The invention relates generally to inspection technology and more specifically, to nondestructive testing techniques using plasma discharge.
Ultrasonic inspection of materials is a commonly used technique for detecting and quantifying material defects and subsurface damage of industrial components. However, one of the limitations of conventional ultrasonic inspection techniques is that a liquid interface between the probe and the material being inspected is necessary to avoid excessive reflection of acoustic energy from the air gap that would otherwise be present at the interface. Non-contact ultrasound inspection is an attractive non-destructive inspection technique, particularly for materials that could be damaged by water, for materials at elevated temperatures, and where the logistics of supplying water at an interface is expensive or difficult. One possible non-contact inspection technique is air-coupled ultrasound. However, air-coupled ultrasound exhibits a relatively poor signal to noise (approximately 40-80 dB less than liquid coupled ultrasound).
Commonly used techniques for generation of ultrasound in materials through air gaps include localized laser heating or ablation of the part surface (laser ultrasound), high power piezoelectric or capacitive membrane devices that are acoustically matched to air, and electromagnetic acoustic transducers (EMATs) that generate mechanical vibrations in the material through electromagnetic force. However, each of these techniques suffers from certain limitations. For example, although laser ultrasound is very effective at generating ultrasound in metals and some composites, it is mildly damaging to the surface of the material and very expensive to implement. As regards conventional high-power air-matched ultrasound transducers and EMATs, it has been observed that these techniques are generally limited in terms of maximum output power.
Hence, there is a need for improved ultrasonic inspection systems that address the aforementioned issues.